Crack retardants are known; however, the prior art disclosures on the use of such known retardants typically require the continuous exposure of the crack path to solutions containing the retardant. The known retardants are of little use when applied to an aircraft because there is no liquid to carry the retardant to the crack tip and fracture surfaces.
In an effort to eliminate the shortcomings of the known crack retardants, the inventors of the present invention disclosed, in U.S. Pat. No. 4,327,152 to Miller et al., assigned to the assignee of the present invention, a protective coating for retarding crack growth in aluminua alloys which requires no liquid to convey the retardant to the fracture surface and at the tip of the advancing crack. The protective coating disclosed by the above-noted patent utilizes a volatile crack growth retardant having a high vapor pressure so that the vapor from the retardant reacts with the surface of the metal to form a protective film. The retardant is blended with a primer, such as an organic paint, and this crack growth retardant reinforced primer is, preferably, covered with a low-permeability organic film so as to prevent the loss of the volatile retardant to the atmosphere.
While the protective coating of Miller et al. overcomes many of the deficiencies of the known crack retardants, there are some drawbacks. The reaction product of hexafluoroisopropanol and cyclohexylamine is very volatile and therefore should be used in conjunction with a low-permeability film as a topcoat in order to prevent loss of the retardant to the atmosphere by evaporation and insure a long-life protective coating. The use of a topcoat is not always desirable or feasible. On the one hand, the topcoat adds weight to the overall load that the aircraft must overcome in order to fly and on the other hand, it is undesirable to apply a topcoat in certain applications, for example, fastener installations.
In studies of actual aircraft structures after periods of normal service, it has been found that approximately 92% of fatigue cracks originate in fastener holes. In order to prevent the passage of moisture and the environment as well as to prevent leaks in pressurized areas, it is customary for most of the fasteners to be installed coated with a liquid sealant which subsequently cures as a tough elastomeric material.
In an attempt to discover a long-life fatigue crack growth retardant for use on fasteners, Miller and Smith tried several less volatile crack retardants which were used to modify the reaction product of hexafluoroisopropanol and cyclohexylamine, hereinafter referred to as the L-8 material. The modified L-8 material was mixed with an elastomeric polymer which happened to be a polysulfide. Much to their surprise Miller and Smith learned that the addition of the modified L-8 material to the polysulfide caused a chemical reaction which stabilized the retardant and made it less volatile. This result was quite unexpected in that one of ordinary skill in the art would expect that the polysulfide polymer, during its curing operation, would have used all the available reaction sites, none being available to react with the modified L-8 material. During further experimentation, it was discovered that the L-8 material could be stabilized by an elastomeric polymer, for example a polysulfide, without the necessity for a less volatile additive.